1. Field of the Invention
This invention relates to audio power amplifiers for driving loudspeakers, and in particular, vacuum tube-type amplifiers. Thirty years after transistor amplifiers became dominant in the marketplace due to their many advantages over vacuum tube amplifiers, there is still substantial interest in vacuum tube amplifiers. Many people feel vacuum tube amplifiers offer superior subjective performance over their solid-state counterparts when used to reproduce music over loudspeakers.
2. Discussion of the Related Technology
One of the biggest problems that has always plagued tube-type amplifiers is that of matching the high-voltage, low current operation of tubes to the low-voltage, high-current requirements of dynamic loudspeakers. The standard solution for this problem is to couple the tubes to the speaker with an output transformer. Winding a transformer that is capable of operating over a wide frequency range is difficult, and the frequency extremes have never been reproduced with low distortion. Furthermore, various parasitic effects cause unpredictable and complex phase shift characteristics that cause stability problems when negative feedback is used. In order to reproduce low frequencies, an output transformer must be large and heavy. Weights of five to ten kilograms are common in high-fidelity applications, and good transformers are expensive.
There are many other problems with output transformers. They become short circuits at frequency extremes. They saturate at low frequencies, and become capacitors at high frequencies. Both effects cause overheating of output tubes. Since output transformers cannot pass direct current (dc), global dc stabilization of multi-stage amplifiers is not possible.
There have been many attempts over the years to eliminate the output transformer. The most successful is what is commonly called OTL, an abbreviation for "output-transformerless," and it is based on placing a sufficient number of tubes in parallel to be able to drive the required current into the loudspeaker. These amplifiers often have ten to twenty or more large-heater type tubes per monaural unit. Even though these amplifiers may have sufficient current to drive the loudspeaker, they are not sufficiently low in output impedance under large signal conditions to properly control back emf from many loudspeakers, and careful matching of amplifier to speaker is required.
A compromise method of solving the problem is to use vacuum tubes in the low-level stages of a power amplifier, and use a transistorized output stage. This type of amplifier has enjoyed some success, but mainly for its relatively low cost. Amplifiers of this type have not won widespread approval from those who prefer tube amplifiers over all solid-state ones.